The Crimean War (1853–1856) was part of a long-running contest between the major European powers for influence over territories of the declining Ottoman Empire.
The war was fought between the Russian Empire on one side and an alliance of France, the United Kingdom, the Kingdom of Sardinia, and the Ottoman Empire on the other. Most of the conflict took place on the Crimean Peninsula, with additional military action in western Turkey and the Baltic Sea region.
The Crimean War is the setting for the flashback in the episode "Night in Question". Both Nick Knight and his master, Lucien LaCroix, are in the Crimea. We do not learn what role the former is playing: unlike LaCroix, he is seen in civilian clothes. The older vampire, however, is posing as a British cavalry officer.
LaCroix's interest in being on scene during the Crimean war is the same as his reason for being at battles in the American Civil War a decade later: he preys on the wounded still lying in the field. These serve him as a convenient source of blood, since their deaths will be attributed to their battle injuries.
After the battle, therefore, LaCroix scours the field for dying soldiers; but, when he finds one still alive, his intentions are recognized. Even as the vampire bites him, the man summons the strength to snatch up a broken piece of wood and stab LaCroix in the heart. The blow is not immediately mortal. Nick brings his master to a ruined house. However, he initially refuses to remove the stake so LaCroix can heal. He does eventually agree to do so, but only in return for the promise that he will be let to go his own way.
In the flashback, LaCroix wears the uniform of a British cavalry officer, quite probably with the 11th Hussars. A contemporary photograph of a junior officer with that regiment shows a very similar uniform.